The present invention relates to an audio amplifier, and more particularly to a low cost multi-purposes audio amplifier.
In a portable communication device, the audio signal may be outputted via different amplifying devices when the portable communication device operates under different modes. The different amplifying devices are arranged to generate audio signal with different sounds respectively. Therefore, the audio subsystem of the portable communication device may comprise various sets of different amplifiers, in which each set of amplifier is arranged to amplify an audio signal with a specific range of power. For example, when a user uses an ear speaker to listen the audio generated by the portable communication device, the amplifier in the audio subsystem used for amplifying the pre-output audio signal may be a Class-AB amplifier. When the user uses a headphone to listen the audio generated by the portable communication device, the amplifier in the audio subsystem used for amplifying the pre-output audio signal may be another Class-AB amplifier. When the user uses a hands-free speaker to listen the audio generated by the portable communication device, the amplifier in the audio subsystem used for amplifying the pre-output audio signal may be a Class-D amplifier or a Class-AB amplifier. Conventionally, the above-mentioned amplifiers are three sets of separated amplifiers in the audio subsystem. Therefore, the above-mentioned amplifiers occupy a large area in the portable communication device. Nowadays, the large size of audio subsystem may greatly reduce the product competitiveness of the portable communication device. Thus, how to reduce the size or hardware requirement of an audio subsystem is an urgent problem in this field.